The present invention relates to an image reading apparatus wherein the size of a median filter is changed to thereby remove noise from the data to be obtained by prescanning.
Image reading apparatus include, for example, microfilm reader-scanners for reading image frames or like original images recorded on microfilm.
To obtain images of optimum density by such microfilm reader-scanners, it is common practice to prescan original images before the main scanning thereof for automatically setting the amount of exposure to be given by a light source to the image for reading (automatic exposure setting, AE), recognizing the image frame (image size) and performing other procedures. For prescanning, data is picked up by thinning out data from the entire image and is then stored in a memory. This is because a memory of great capacity and a prolonged period of processing time are required for manipulating the entire image data. The data picked up and stored by the prescanning is used for determining reading parameters (such as the amount of exposure, variation in magnification, etc.) which are optimum for the main scanning.
In the case where the optimum parameters for the main scanning are determined based on the thinned-out data which is picked up, the decrease in the number of data items picked up leads to a corresponding reduction in the processing time, whereas the picked-up data, if containing noise, results in a higher probability of determining erroneous parameters and a likelihood of exerting an adverse effect on the determination of parameters. Accordingly, there is a need to remove the noise from the data to be picked up as thinned out. With the conventional method of removing noise, an increased number of data items are picked up to lessen the probability of the picked-up data containing noise, and the average value of the picked-up data items is calculated. However, the conventional method of removing noise by the software of calculating the average value takes time to calculate the average value, and the analysis of the prescanning data requires an increased period of time for this reason and also because of the increase in the number of image data items to be sampled, consequently entailing the likelihood of retarding the operation of the scanner itself.